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digitalmars.D.learn - alias this + std.array.popFront

reply "Tobias Pankrath" <tobias pankrath.net> writes:
Hello,

the following struct is not an input range because you can't call 
popFront on it, however I'd say you should be able to.

struct Wrapper {
     int[] _data;
     alias _data this;
}

Bug or intended behaviour?

Thank you!
Mar 29 2013
parent reply "Tobias Pankrath" <tobias pankrath.net> writes:
On Friday, 29 March 2013 at 22:13:31 UTC, Tobias Pankrath wrote:

 Hello,

 the following struct is not an input range because you can't 
 call popFront on it, however I'd say you should be able to.

 struct Wrapper {
     int[] _data;
     alias _data this;
 }

 Bug or intended behaviour?

 Thank you!



The reason seems to be that, while is(DynamicArrayTypeOf!Wrapper 
== int[]) holds, isDynamicArray!Wrapper equals false, because 
isDynamicArray checks for !isAggregateType!Wrapper. Looks like a 
bug isDynamicArray to me.
Mar 29 2013
parent reply "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Friday, March 29, 2013 23:22:36 Tobias Pankrath wrote:

 On Friday, 29 March 2013 at 22:13:31 UTC, Tobias Pankrath wrote:

 Hello,
 
 the following struct is not an input range because you can't
 call popFront on it, however I'd say you should be able to.
 
 struct Wrapper {
 
 int[] _data;
 alias _data this;
 
 }
 
 Bug or intended behaviour?
 
 Thank you!


 
 The reason seems to be that, while is(DynamicArrayTypeOf!Wrapper
 == int[]) holds, isDynamicArray!Wrapper equals false, because
 isDynamicArray checks for !isAggregateType!Wrapper. Looks like a
 bug isDynamicArray to me.


No, it is most definitely _not_ a bug in isDynamicArray. The isX traits in 
std.traits specifically check for exact tyes and _not_ implicit conversions. If 
it's a bug, it's in the fact that isDynamicArray is used in popFront's 
template constraint. Whether or not Wrapper should be treated as a range is up 
for some debate (implicit conversion tends to cause big problems with template 
constraints, as it becomes far too easy to have constraints which pass and yet 
the function doesn't actually work with the type), but the main problem is 
that front and popFront for arrays don't agree on what they should work with - 
front takes T[] and popFront checks isDynamicArray. So, front works with 
implicit conversion, and popFront doesn't, and that needs to be fixed. But the 
bug is in front/popFront, not isDynamicArray.

I believe that there was a thread on this recently (started by monarch_dodra 
IIRC), so you can look for that discussion for more information if you'd like.

- Jonathan M Davis
Mar 29 2013
next sibling parent "Tobias Pankrath" <tobias pankrath.net> writes:
On Saturday, 30 March 2013 at 00:23:17 UTC, Jonathan M Davis 
wrote:

 On Friday, March 29, 2013 23:22:36 Tobias Pankrath wrote:

 On Friday, 29 March 2013 at 22:13:31 UTC, Tobias Pankrath 
 wrote:

 Hello,
 
 the following struct is not an input range because you can't
 call popFront on it, however I'd say you should be able to.
 
 struct Wrapper {
 
 int[] _data;
 alias _data this;
 
 }
 
 Bug or intended behaviour?
 
 Thank you!


 
 The reason seems to be that, while 
 is(DynamicArrayTypeOf!Wrapper
 == int[]) holds, isDynamicArray!Wrapper equals false, because
 isDynamicArray checks for !isAggregateType!Wrapper. Looks like 
 a
 bug isDynamicArray to me.


 No, it is most definitely _not_ a bug in isDynamicArray. The 
 isX traits in
 std.traits specifically check for exact tyes and _not_ implicit 
 conversions. If
 it's a bug, it's in the fact that isDynamicArray is used in 
 popFront's
 template constraint. Whether or not Wrapper should be treated 
 as a range is up
 for some debate (implicit conversion tends to cause big 
 problems with template
 constraints, as it becomes far too easy to have constraints 
 which pass and yet
 the function doesn't actually work with the type), but the main 
 problem is
 that front and popFront for arrays don't agree on what they 
 should work with -
 front takes T[] and popFront checks isDynamicArray. So, front 
 works with
 implicit conversion, and popFront doesn't, and that needs to be 
 fixed. But the
 bug is in front/popFront, not isDynamicArray.

 I believe that there was a thread on this recently (started by 
 monarch_dodra
 IIRC), so you can look for that discussion for more information 
 if you'd like.

 - Jonathan M Davis


Yes, you are most probably right that the bug is not in 
isDynamicArray. However DynamicArrayTypeOf and isDynamicArray are 
rather inconsistent since DynamicArrayTypeOf does not check exact 
types.
Mar 30 2013
prev sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Saturday, 30 March 2013 at 00:23:17 UTC, Jonathan M Davis 
wrote:

 I believe that there was a thread on this recently (started by 
 monarch_dodra
 IIRC), so you can look for that discussion for more information 
 if you'd like.


http://forum.dlang.org/thread/xhzojyrrcidusvreupsc forum.dlang.org

"alias this and array popFront: is this a bug?"

The similarity in code snippet is uncanny. Kenji seems to have 
jumped on the issue, and fixed it exactly according to suggestion.
Mar 30 2013